A Wavelength Switched Optical Network (WSON) supports end-to-end optical paths, between nodes requiring connection in the network. Intermediate nodes in this type of network support wavelength switching and may also support wavelength conversion. In contrast with point-to-point optical communication links which provide high-capacity transport, always between the same pair of nodes, a WSON supports the setting up and tearing down of paths between pairs of nodes of a network having a more complex topology, such as a ring, interconnected rings or mesh topology. A Routing and Wavelength Assignment (RWA) function is typically carried out by a path computation element PCE of the WSON and involves routing a path across the WSON and assigning a wavelength to the path.
WSON is a standardization effort of the Internet Engineering Task Force (IETF). WSON is the application of a GMPLS based control plane and PCE to an “all optical” network, that is, no electrical switching is part of the WSON scope. The state of the art was a network landscape based on multi-degree ROADMs with the ability to switch (for recovery purposes) in few minutes on alternative paths defined in a planning phase.
For any restoration to use an alternative path to avoid a fault, two options are in theory on the table: Pre-Planned (PP) and On-The-Fly (OTF). In the PP option, computation of the alternate path is performed before service delivery, therefore the computation is not time-critical: this allows long and fully accurate computations to be performed, important when considering optical impairments which limit the length of paths. The computation not being time-critical allows its execution in an off-line dedicated computation element, with fully detailed network information. Computation may be performed together with network design, including hardware equipment requirement definition: this is needed if paths require regeneration. Once computed, the recovery path is reserved in the network. This has two main advantages: recovery resources cannot be “stolen” by any other user and path computation time does not affect recovery time.
In the OTF option, computation is time-critical, as its time adds to the restoration time: this requires computation and information to be summarized. Time-criticality also recommends the computation to be performed close to the network, to avoid communication overhead: for this reason, OTF path computation is normally performed in a distributed environment in the network nodes, with summarized information and limited visibility, which may also lead to resource conflict.
Typically network operators see the OTF as enabling reduced CAPEX compared to implementing PP restoration for multiple faults since PP requires so much spare capacity to be installed and reserved. Nevertheless, currently for GMPLS controlled wavelength switched optical networks, pre-planned protection paths are still preferred because the chance is too high that an OTF computation will fail to find any alternative path. This is because of the need for wavelength continuity and the limitations of optical reach, since it is too expensive to have wavelength conversion or optical regeneration throughout the network.
Two known types of PP recovery schemes for optical networks are Safe-OSNCP and Transponder Sharing Protection (TSP). Safe-OSNCP is based on a classical OSNCP, with an extra preplanned path. There is a working path which uses a first set of transponders at the source and destination nodes for conversion between electrical and optical domains. A first preplanned alternative path uses different transponders and a different route through the nodes. Electrical switches are provided to move the traffic flow onto the alternative path if needed. The extra preplanned path uses the same transponders as the first alternative path and ensures there is protection after a second fault.
Transponder sharing protection has only the working path and a set of two or more preplanned alternative paths which can protect to multiple faults. The pre-planned paths are not signaled on the WSON network, they are in an “idle” state and stored in the NEs.
WSON evolution towards On-The-Fly can be regarded as a substitute for the Transponder Sharing protection: only the worker path is active on the NEs and when a failure occurs, the NMS calculates a new route, deletes the current route and activates the new one on the NEs. In practice the new route is sent only to the source node, not to all the nodes. The Safe-ONSCP and TSP schemas are “static” and, especially for TSP, resource consuming as the NE has to store a lot of information. The resource consuming problem is mitigated by the new “On the Fly” but there remains a problem in the delay before the new route can be calculated.